Applications such as semiconductor fabrication processing increasingly require more accurate measurements and quicker and more consistency in timing in the delivery of gases from components such as a mass flow controller (MFC).
FIG. 1 is a schematic diagram illustrating an MFC 100, according to an embodiment of the prior art. Generally, MFC 100 is a device used to measure and control the flow of fluids and gases. MFC 100 has an inlet port 110, an outlet port 120, a mass flow sensor 130, a flow bypass 135, and a control valve 140. The control valve 140 is adjusted in accordance with measurements from the mass flow sensor 130 in order to achieve a desired gas flow. The mass flow sensor 130 can be a thermal sensor, allowing the mass flow to be measured by sensing a temperature profile between the “no flow” and the “at flow” conditions.
Problematically, typical in MFCs using a thermal sensor to measure mass flow, measurements can be inaccurate during inlet pressure transients, because gas flow is measured and maintained at the thermal sensor 130 which is at the inlet of the MFC 100 rather than at the outlet port 120 where gas exits. The changing inlet pressure to the MFC causes the flow into the MFC 100 to be different than the flow out of the MFC outlet port as additional mass enters the MFC 100 to pressurize the volume between the thermal sensor and the downstream valve seat.
Although a pressure based MFC (or low flow injector) eliminates the measurement location issue by locating a characterized flow restrictor at an outlet port, thus allowing flow measurement at the outlet of the MFC (or injector), gas delivery, from both pressure based and thermal based MFC, can suffer from slug flow at low flows.
FIG. 2 is a schematic diagram illustrating a system 200 with a pressure based MFC 210 to deliver a low flow gas, thus eliminating pressure issues, according to an embodiment of the prior art. A higher flow MFC 220 delivers a carrier gas that mixes, at the tee where conduits 225 and 215 meet, with the low flow gas to speed up delivery through the system 200. Ideally, the low flow gas of conduit 215 mixes with the higher flow gas of conduit 225 for a desired mixture of gases.
However, a mass flow of the higher flow gas races through the conduit 225 and pressurizes and filling the conduit 225 and the majority of the conduit 215. This pressurization at the onset of gas flow occurs as the flow of gas encounters the inherit flow resistance of the “downstream plumbing” a differential pressure is needed to drive flow. Before the low flow gas can reach the tee where conduits 225 and 215 meet and can mix with the carrier gas, sufficient mass must flow from MF 210 to displace the carrier gas that has partially filled conduit 215 at the onset of the gas flows. The time required for this displacement is roughly equal to the mass of the carrier gas in 215 divided by the flow rate of the low flow gas form 210. The author defines this time delay as a “slug flow” delay as the slug of carrier gas in 215 must be displaced. For flows from 210 of magnitude 10 sccm (standard cubic centimeters per minute) required slug flow delays of 5 to 15 seconds are typical in conventional systems. Delays longer than 1 minute are possible for a 1 sccm flow. These delays are unseen by instrumentation and unknown by many users however it is standard practice to delay “processing” for a period of time after gas flow have begun. Such delays on expensive equipment limit throughput and thus drive up the cost of the product being produces. As a consequence, delivery of the low flow gas into the mixture can be delayed beyond a tolerance of the process and the slug flow delay time can vary depending on the varying volume of the components of different suppliers use in a system.
Additionally, a pressure based MFC can suffer from slow bleed downs. A volume existing between a flow restrictor and an upstream valve seat controlling pressure to the flow restrictor contains a bleed down mass. When an MFC is instructed to stop gas flow, the upstream valve seat is closed, but gas continues to flow through the flow restrictor as the bleed down mass exits. Bleed down is a function of conductance of the flow restrictor. Larger restrictors with larger conductance can be used to speed up the bleed down time, but the tradeoff can be a significant increase in drift and inaccuracy.
What is needed is a robust technique to provide accurate measurements at a point of gas delivery, while minimizing slug flow and bleed down times.